S=1.2+2.3+3.4+...+49.50
=> 3S=1.2.3+2.3.3+3.4.3+...+49.50.3
=1.2.3+2.3.(4-1)+3.4.(5-2)+...+49.50.(51-48)
=1.2.3+2.3.4 - 1.2.3+3.4.5 - 2.3.4+...+49.50.51 - 48.49.50
=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...+(47.48.49-47.48.49)+49.50.51
=124950
==> S= 124950:3=41650